Production instrument for producing compound semiconductor quantum boxes and light emitting devices using those quantum boxes

ABSTRACT

A method and instrument are provided for producing compound semiconductor crystallized ultrafine particles of Group II-VI or Groups III-V making use of a vapor phase reaction of an element of Group II or III with an element of Group IV or V. Also disclosed is a gain modulation type quantum box laser element of high performance constructed by geometrically disposing the ultrafine particles.

This is a divisional of application Ser. No. 07/786,936 filed Nov. 4,1991 (U.S. Pat. No. 5,229,170) which is a divisional of application Ser.No. 07/367,570 filed Jun. 19, 1989 (U.S. Pat. No. 5,079,186).

BACKGROUND OF THE INVENTION

Recently, ultrafine particles (diameter˜100 Å) have been vigorouslystudied and developed from both a purely academic aspect forexperimentally solving the Kubo effect and the like and an industrialaspect for utilizing them as ornaments, magnetic memory elements and thelike [Hayashi; J. Vac. Sci. Technol. A5(4), 1375 (1987)]. Materialsthereof, however, are limited to simple metals such as gold, iron andthe like or stable oxides such as alumina and the like, and thus thereis no example indicating that ultrafine particles of compoundsemiconductors are produced. While metallic ultrafine particles areobtained by thermally evaporating metal in the atmosphere of an inertgas such as an argon gas and the like (pressure˜1Torr) wherein a meanfree path is restricted, the inert gas only restricts the mean free pathof metallic atoms and is not taken into the ultrafine particles in thiscase.

While most attempts to produce ultrafine particles (quantum boxes) ofthe compound semiconductors have encountered severe difficulties and theprospects for realizing such particles are very dim, there are someprospective techniques being investigated. One of them is a method tocombine an ultra thin film epitaxial growth method such as MolecularBeam Epitaxy (MBE), metalorganic Vapor Phase Epitaxy (MOVPE) and thelike with a local processing method (etching, doping, disordering)effected by Focused Ion Beam (FIB). There is also a potential techniquewherein a selective wet etching is used in place of the FIB processing.In any case, while this method is to process one dimensionally quantizedstructure layered by the MBE to expand a quantized dimension, achievingprocessing to an accuracy of˜100 A is yet difficult and greatlydependent on a development to be achieved hereinafter. Another methodlocally deposits compound semiconductors directly or indirectly on asuitable substrate using thin needle electrodes. This method which seemsflexible and attractive at first glance has a problem in reliability andreproducibility because a substrate and a temperature causing asuccessful epitaxial growth must be selected in practice and further thethin electrodes must be operated under severe epitaxy conditions.

With respect to a handling method of ultrafine particles, a prior artmanipulator is in its primitive stage. There is of course no embodimentfor handling ultrafine particles of compound semiconductors and metallicultrafine particles are only handled by such methods that fine particlesevaporated in the above inert gas and deposited on a wall are gatheredby a brush or the fine particles are transported onto a sample stage ofan electron microscope through a micro-jet stream.

Zealous developments are in progress to improve the performance anddegree of integration of a semiconductor laser. At present, however, aquantum well laser making use of a one dimensionally quantized andlayered thin film structure cannot provide excellent threshold currentdensity, line width and temperature characteristics because quantizationis not effected in a direction parallel to the thin film and defectssuch as steps exist. There are some methods for reducing interfacedefects such as by carefully controlling conditions for a thin filmgrowth effected by the MBE or the like. If an ideal interface isrealized, the characteristics of laser will be improved as necessary.The fact, however, that the quantization is not affected in thedirection parallel to the thin film as a principle must remain as afactor for reducing the laser characteristics.

With respect to the integration of a semiconductor laser, a generalstructure including a Fabry-Perot type resonator is not basicallysuitable. There is an example wherein a mirror face not inferior to acleaved facet is created on a substrate by reactive ion beam etching anda Fabry-Perot resonator is fabricated using it. Nevertheless, it isapparent that a stepped structure caused by this processing is a largeobstacle to the integration of other elements. A prototype laser[Distributed Feedback (DFB) laser] provided with an embedded diffractiongrating in place of a reflection edge is fabricated with advantageouscharacteristics. A usual DFB laser, however, only feeds back light of aparticular wavelength diffused from a diffraction grating and uses it tocontrol an oscillation mode, and thus gain is not modulated.

Since three dimensional quantum boxes have a very sharp discrete energylevel, the application thereof to the active layer of a semiconductorlaser can provide high performance laser. To obtain the threedimensional quantum boxes, howoever, ultrafine particles with a diameterof hundreds of Angstroms or less must be prepared. It is very difficultto produce this size of ultrafine particles of compound semiconductorsof Groups III-V (or Groups II-VI), and then none of the test making useof the FIB processing has been successful. Further, even if theseultrafine particles are produced, a method for sizing and handling thembased on a new principle and a manipulator are required because theirsize is too small.

A compound semiconductor laser is integrated (opto-electronic integratedcircuit: OEIC) to be used in communication, data processing,opto-computer applications and the like. While an example of a prototypeOEIC is reported, the degree of integration thereof is still very lowand said to be retarded more than ten years than a Si technology. Toincrease the degree of integration of the OEIC, the semiconductor lasermust be arranged to a structure without a Fabry-Perot type resonator. ADFB laser aiming at single mode oscillation, narrow line width andimproved temperature characteristics in addition to the abovearrangement is being developed. While the DFB laser does not require anedge mirror, it cannot yet provide micro laser and improved lasercharacteristics at the present stage.

SUMMARY OF THE INVENTION

The present invention provides a production method of compoundsemiconductor quantum boxes, a production instrument thereof, a handlingmethod of ultrafine particles such as the compound semiconductor quantumboxes and the like, and light emitting devices using the quantum boxes.According to the present invention, the method of producing compoundsemiconductor quantum boxes comprises the steps of evaporating anelement of Group III or II in a pressurized space controlled by a vaporof an element of Group V or VI and producing the compound semiconductorquantum boxes of Groups III-V or Groups II-VI making use of reaction ina vapor phase. In addition, the production instrument of the compoundsemiconductor quantum boxes comprises a partial pressure control devicefor controlling a partial pressure of an element of Group V or VI, anevaporation rate control device for controlling an evaporation rate ofan element of Group III or II, and a sizing and storing device forsizing and storing particles of the produced quantum boxes for producingthe compound semiconductor quantum boxes of Groups III-V or Groups II-VImaking use of a vapor phase reaction according to the above-mentionedprocess.

Further, a method of handling the ultrafine particles such as thecompound semiconductor quantum boxes comprises the steps of chargingrespective ultrafine particles and causing an electrostatic force to acton the ultrafine particles by regulating a voltage imposed on needleelectrodes for transporting the ultrafine particles to any arbitrarylocation and disposing the same there. In addition, the presentinvention provides a light emitting device which comprises the compoundsemiconductor quantum boxes disposed one dimensionally, twodimensionally or three dimensionally in synchronism with a wavelength ofemitted light for constructing a gain modulation type laser device.

According to the present invention, the crystallized ultrafine particlesare produced making use of a vapor phase reaction of the element ofGroup II or III with the element of Group IV or V, and the ultrafineparticles can be sized, stored, transported and disposed by using anelectrostatic (magnetic) force, respectively. Further, a gain modulationtype laser element of high performance can be realized by using, as theactive layer thereof, the quantum boxes which are composed of theultrafine particles of the compound semiconductors prepared by themethod and instrument of the present invention and disposed insynchronism with the wavelength of emitted light.

The present invention intends to apply the compound semiconductorultrafine particles to the active layer of a light emitting device asquantum boxes in the field of opto-electronics. Widely recognized is apossibility that a device having a new function may be obtained by usinga quantum effect eminently appearing when the active layer is subjectedto a sub-micron processing. Conventional processing techniques, however,are very difficult to prepare a material (quantum boxes) which isquantized up to three dimension and only produce a layered thin filmstructure quantized one dimensionally by a molecular beam epitaxy (MBE)at best. The present invention overcomes this drawback, greatlyincreases a degree of freedom for producing quantum boxes, whereby asemiconductor laser diode of high performance having low thresholdcurrent value, narrow line width and improved temperaturecharacteristics. Further, the disposition of the quantum boxes as theactive layer of the laser device in synchronism with the wavelength ofthe emitted light enables laser oscillation to be effected in thedirection of the disposition of the quantum boxes without the need forthe resonator, which contributes to the integration of the semiconductorlaser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a production instrument of ultrafineparticles of compound semiconductors making use of a gas phase reactionaccording to the present invention;

FIG. 2 is a partly cross sectional view of a ultrafine particlemanipulator according to the present invention;

FIGS. 3(A) and 3(B) are a plan view of a gain modulation type laserdiode using compound semiconductor quantum boxes and a cross sectionalview thereof taken along line X-X' in FIG. 3(A);

FIG. 4 is a plan view of a gain modulation type laser diode;

FIG. 5 is a partly perspective view of a gain modulation type laser (anoptical pumping type); and

FIGS. 6(a) through 6(d) are diagrams explanatory of a method ofproducing devices shown in FIGS. 3 and 4.

The following reference numerals are used throughout the drawings: 1 . .. arsenic vapor source, 2 . . . gallium molecular beam source, 3 . . .reaction chamber, 4 . . . fine particle charging device, 5 . . . chargedfine particle converging device, 6 . . . electrostatic deflector, 7 . .. fine particle storing device, 8 . . . particle takingin anddifferential pumping device, 11 . . . charged ultrafine particles, 12 .. . unconductive type storing plate, 13 . . . needle electrode, 14 . . .insulating film, 21 . . . n type substrate, 22 . . . n type clad layer,23 . . . current block layer, 24 . . . compound semiconductor quantumboxes, 25 . . . p type clad layer, 26 . . . cap layer

DETAILED DESCRIPTION OF THE INVENTION

A means, operation and embodiments according to the present inventionwill be described below with reference to drawings.

FIG. 1 shows a general idea for a method of producing ultrafineparticles of compound semiconductors making use of a gas phase reactionand a schematic arrangement of an instrument for the method. Descriptionwill be made with reference to the production of ultrafine particles ofGaAs as an example. Designated at 1 is an arsenic vapor sourcecontaining metallic arsenic. When the temperature of the vapor source 1is regulated in the range of 200° C. to 500° C., the vapor pressure ofthe arsenic in the reaction chamber can be changed in the range of 10⁻⁴Torr to 100 Torr. At the time, the temperature of the reaction chamber 3is increased by a heater to a temperature substantially similar to thetemperature of the arsenic vapor source 1 to prevent the arsenic vaporfrom condensing on a wall of the chamber. Designated at 2 is a Gamolecular beam source and essentially the same as a Knudesen cell(K-cell) used in a usual MBE device. This source operates in the rangeof 800° C. to 1200° C. and supplies a Ga atom beam at 10¹⁴ to 10¹⁷atom/cm².sec. Note that in FIG. 1 while both the As source and the Gasource are composed of a solid source used in the usual MBE, they may becomposed of a gas source such as arsine (AsH₃), tri-methyl gallium[Ga(CH₃)₃ ] and the like in place of the solid source. The reactionchamber 3 is provided with an ultrahigh vacuum pumping system capable ofpumping background pressure to 10⁻¹¹ Torr or less to prevent impuritiesfrom being mixed with ultrafine particles.

Ga atoms charged in the reaction chamber 3 from the Ga source 2 collideagainst arsenic molecules (mixture of As, AS₂ and AS₄) and react withthem to form GaAs_(n) molecules (n=1˜4). These molecules (in particular,molecules rich with the number of As atoms) have a low probability tocombine with arsenic molecules next and high probability to combine withGa atoms from a view point of energy. When they combine with the Gaatoms once, they are liable to combine with arsenic molecules on thecontrary. This tendency is derived from that the combination of Ga andAs has an ion property to a certain degree. The GaAsn molecules producedfirst repeat collision and reaction with Ga and Asn molecules many timesto grow into GaAs fine crystal particles. As easily presumed from thisreaction mechanism, the growing rate of the fine crystals depends on themean free path of molecules, i.e., the density of the Ga and Asnmolecules in the reaction chamber. Then, the regulation of them (bychanging the temperature of the K-cell and the flow rate of the gas) canchange the growing rate, i.e., the size of the fine particles producedin a wide range. Note that although not shown in FIG. 1, the reactioncan be accelerated, for example, by a laser beam irradiated at thevicinity of the central portion of the reaction chamber to thermally oroptically excite the molecules.

The GaAs fine particles produced as described above fall to the lowerportion of the reaction chamber due to gravity while they continuouslygrow. Since the reaction chamber has a limited volume, the size of thefine particles has a certain distribution. In FIG. 1, reference numerals4 through 8 schematically show a method and devices for sizing andstoring the produced fine particles, wherein 8 designates a fineparticle taking-in device also serving as an orifice for differentiallypumping the reaction chamber 3 and fine particle sizing/storing devices4-7. This differential pumping system enables the sizing/storing devicesto operate in high vacuum (≲10⁻⁵ Torr) against the high pressure (≲100Torr) in the reaction chamber. Designated at 4 is a fine particlecharging device including an electron source such as a tungsten filamentand the like and an anode, 5 designates an electrostatic or magneticlens device for converging charged fine particles, 6 designates anelectrostatic deflector for giving a horizontal moment to the chargedfine particles. The charged fine particles passing through theelectrostatic deflector 6 are sized and stored in the fine particlestoring device 7 with each size, drawing the parabolic trajectorydetermined by charging state and mass (size). The storing device 7 isprovided with a multiplicity of partitions and a multiplicity ofrespectively independent storing plates 12. Two kinds of these storingplates 12 are prepared, one of them being composed of a conductivematerial and the other being composed of an unconductive material. Theconductive type storing plate 12 is used to monitor the amount of thefine particles produced (number) and the producing rate thereof by themeasurement of the electric pulse of charged fine particles fallingthereon. The unconductive type plate 12 is used to preserve the chargedstate of the charged fine particles and enable the particles to behandled hereinafter. These two kinds of storing plates can be disposedside by side for simultaneous use. Further, since the charged state ofthe fine particles is preserved in the unconductive type storing plate,repulsion arises among the fine particles and thus the fine particlesdisperse and fall to the storing plates, which is advantageous for thehandling the fine particles hereinafter.

In FIG. 1, 210 through 270 designate the fine particles sized and storedin the storing plates 12. The fine particles 11 passing through theelectrostatic reflector 6 has a trajectory which is determined by boththe gravity acting downward dependent on the size (mass) of theparticles and the electrostatic force acting horizontally dependent onthe charged state of the particles. Therefore, the regulation of thevoltage imposed on the electrodes of the electrostatic reflector 6enables the particles having desired sizes to be stored in the storingplates 12. In FIG. 1, the respective particles have sizes asfollows:210=20A or less, 220=20˜50A, 230=50˜80A, 240=80˜100A,250=100˜120A, 260=150˜160A, and 270=200A or more.

While these fine particles 210˜270 produced and stored as describedabove are used in the active region of various devices as quantum boxeshaving specific property, usual methods of taking-out, transportationand disposition are not of course applicable because they have a veryfine size. Usual methods of taking-out, transportation and dispositionof ultrafine particles will be described below.

FIG. 2 is a diagram illustrative of a principle for transporting anddisposing the ultrafine particles to an arbitrary location, wherein 11designates the charged ultrafine particles placed on the unconductivetype storing plate 12, 13 designates a needle shaped electrode composedof tungsten, platinum or the like having a radius of curvature ofabout˜0.1 μm at the distal end thereof, 14 designates an insulating filmcomposed of SiO₂ or the like and deposited on the surface of the needleelectrode 13 by a method such as a plasma chemical vapor deposition(CVD) or the like, the film having a thickness of 100 Å to 0.1 μm. Theneedle electrode can be three-dimensionally driven in vacuum by a fineregulating unit using a Piezo-electric device and a rough regulatingunit such as a vacuum bellows or a spring like those used in amicroscope device well known recently as a scanning tunneling microscopy(STM) [for example, S. T. Tang, J. Boker, and R. H. Storz: Appl. Phys.Lett. 52(3), 1988, 188].

When a positive voltage of about 1 mV-1 V is imposed on the needleelectrode 13 and the distal end thereof is approached to a pointhundreds of angstrom to 0.1 μm apart from the charged particles, anelectrostatic attractive force acts on both the needle electrode and thefine particles and thus the fine particles are left from the storingplate 12 and deposited on the surface of the insulating film 14. In thisoperation, caution is necessary because when the radius of curvature ofthe distal end of the needle electrode 13 is made excessively fine as isused in a usual STM (≲100 Å), or the thickness of the insulating film 14is made excessively thin (≲100 Å), the movement of charges are caused bythe tunneling and the action of the electrostatic force disappears.

The fine particles deposited on the distal end of the needle electrodecan be transported to any arbitrary location by the drive mechanism ofthe needle electrode. In order to dispose the fine particles on thedesired location of a desired substrate, the fine particles are firstmoved to a location closely adjacent to the surface (˜100 Å) of thedesired location by the drive mechanism of the needle electrode and thenthe needle electrode is coupled to ground to remove the action of theelectrostatic force. Alternatively, a little reverse voltage is imposedon the needle electrode to apply a repulsion to the fine particles. Notethat this handling method is applicable to any arbitrary ultrafineparticles such as metal semiconductor and insulator.

FIG. 3 shows one of embodiments wherein GaAs quantum boxes are appliedto the active layer of a semiconductor laser with a short wavelength.FIG. 3(A) is a plan view of the active layer and FIG. 3(B) is a crosssectional view of the structure of the semiconductor laser taken alongline X-X' of FIG. 3(A). In FIG. 3(B), 21 designates an n type GaAssubstrate, 22 and 25 designate n and p type Al_(x) Ga_(1-x) As(x=0.3˜0.6) clad layer, respectively (with a thickness of about≲1 μm,respectively), 23 designates a current block layer of SiO₂ or the like(3/8100 Å), 24 designates GaAs quantum boxes with a size of about˜150Å), 26 designates a p type GaAs cap layer, and 27 and 28 designatemetallic electrodes. The GaAs boxes of3/8100 pieces are disposed in linein the X-X' direction with intervals a expressed as follows.

    a=N·λ/n

where N is a positive integer, λ is a wavelength of emitted light (inair) (6000˜8000 Å), and n is a refraction factor of media. With thisarrangement, a laser oscillated light beam having a sharp spectrum inthe X-X' direction determined by the constant a can be obtained. Thelaser beam has a low oscillated threshold current because of highquantum level state density and the oscillation wavelength thereof isvery stable to temperature change because it is doubly restricted by theenergy value of a quantum level and a.

FIG. 4 is an embodiment of a semiconductor laser having the active layerof GaAs quantum boxes 24 cyclically disposed two dimensionally. Thecross sectional arrangement thereof is the same as that shown in FIG.3(B). When both cycles a and b are given by integral multiplex of λ/n, amultiple laser beam from which output light beams can be taken in the Xand Y directions and the like is obtained. When the cycle in any onedirection is given by the integral multiples of λ/n, an output lightbeam is obtained in that direction only.

A method of producing the devices shown in FIGS. 3 and 4 will bedescribed here in detail (refer to FIG. 6). First, an n type Al₀.3 Ga₀.7As (carrier density of 1×10¹⁸ cm⁻³) layer 22 with a thickness ofabout˜0.5 μm is grown on an n type GaAs substrate (carrier density of1×10¹⁸ cm⁻³) 21 by a usual epitaxial growth such as MBE, MOCVD or thelike (FIG. 6-a). The above GaAs fine particles 24 (quantum boxes) with adiameter of˜150 Å are disposed on the layer 22 in a one-dimensional linestructure or in a two dimensional network structure using the abovemethods of taking-out, transportation and disposition (FIG. 6-b). Next,an SiO₂ film with a thickness of˜150 Å is deposited by a usual thin filmdeposition method such as a sputter deposition or the like and thequantum boxes are filled with the film (FIG. 6-c), wherein a selectivegrowth condition is selected to enable the SiO₂ film to be depositedonly on the AlGaAs and not to be deposited on the GaAs. Next, p typeAl₀.3 Ga₀.7 As and p type GaAs (with a thickness of ˜0.5 μm and 0.1 μm,respectively, and carrier density of 1×10¹⁸ cm⁻³ for both materials) aresequentially grown by epitaxy (FIG. 6-d). While the epitaxial growth ofthe p type AlGaAs may be though difficult because almost all of thesubstrate is covered by the amorphous SiO₂, this understanding is notcorrect. More specifically, since the GaAs quantum boxes appearing atplaces on the surface of the substrate are composed of mono-crystalswhich act as seeds to cause a phenomenon called seed epitaxy, the seedsare grown to crystals of good quality at least at the vicinity of theseeds. While a region far apart from the seeds has inferiorcrystallinity and thus high resistance, this region is originally aregion where a current block layer (SiO₂) exists and no serious obstacleis caused. After the process in FIG. 6-d is finished, electrodes aremounted by a process employed in the fabrication of a usualsemiconductor laser to complete a device.

A method of producing a device with the quantum boxes disposed threedimensionally is the same as that described above. A device produced byalternately using a usual thin film deposition method and the abovemethod of taking-out, transportation and deposition of the fineparticles will be described below.

It is difficult to realize a current injection exciting type laser usingan arrangement of GaAs quantum boxes cyclically disposed threedimensionally. When media are composed of an optical pumping typematerial such as SiO₂, ITO (indium-tin oxide) or the like which istransparent to light and exited by suitable light, distinguished laserof high performance is obtained. FIG. 5 shows an embodiment of it,wherein the GaAs quantum boxes 24 are cyclically embedded in media 30 ofSiO₂ three dimensionally. All or a part of cycles a, b and c is given byintegral multiples of λ/n. When this arrangement is exited byirradiating suitable light (with a wavelength shorter than the intervalsof a quantum well level) from the outside, multi-beam oscillation (orsingle beam) is obtained.

Note that while GaAs quantum boxes are used in the above embodiments,other quantum boxes of compound semiconductors of Groups III-V or GroupsII-VI such as InP may be used. A production method and handling of thesequantum boxes must be based on the present invention and othercomponents (semiconductor thin films, metallic electrodes and the like)thereof must be prepared by a usual method such as MBE.

According to the present invention, compound semiconductor quantum boxesof which production has been conventionally very difficult even by thelatest sub-micron lithography techniques can be very easily produced ona large scale. Since the quantum level of the quantum boxes isdetermined by the size thereof, it is important to prepare various sizesto use the quantum boxes as the active layer of a laser element.According to the present invention, ultrafine particles of various sizescan be sized, stored, transported and disposed with each size thereof.This method is applicable not only to the ultrafine particles of thecompound semiconductors but also to metallic ultrafine particles ofiron, nickel and the like which are already produced on an industrialscale and ultrafine particles of any other arbitrary materials. Further,a laser element using, as the active layer thereof, the quantum boxescomposed of the ultrafine particles of the compound semiconductors anddisposed in synchronism with the wavelength of emitted light can providea multi-beam laser element having a low threshold value and narrow linewidth which are required in the fields of communications, measuringinstruments, computers and the like.

What is claimed:
 1. A light emitting device comprising a light emittingsource including a plurality of compound semiconductor quantum boxesdisposed on a substrate at an interval of "a" given by the followingequation:

    a=N·λ/n

wherein N is a positive integer, λ is a wavelength of emitted light ofsaid device and n is a media refraction factor, a series of currentblock layers which surrounds said quantum boxes, and clad layers whichsandwich said quantum boxes and current block layers, said current blocklayers being made of SiO₂.
 2. The light emitting device of claim 1,wherein said substrate is a compound semiconductor substrate.